5.5 Collision Model

Collisions must be effective

The collision model states that reactions occur when particles collide with sufficient energy and correct orientation. Not every collision leads to a reaction — only ones that meet or exceed the activation energy (Ea) and have proper geometric alignment result in successful bond breaking and forming.

Activation energy and temperature

Activation energy (Ea) is the minimum energy needed for a collision to be successful. Raising temperature increases the fraction of molecules with kinetic energy ≥ Ea (shown by the Maxwell-Boltzmann distribution shifting right), increasing collision frequency and successful collisions, which speeds up the reaction rate.

Arrhenius equation ties it together

The Arrhenius equation, k = Ae^(-Ea/RT), links rate constant k to temperature T and Ea. A is the frequency factor, accounting for collision frequency and orientation. A linearized form, ln(k) = -Ea/R(1/T) + ln(A), lets you find Ea graphically from a plot of ln(k) vs 1/T.

Key terms

Collision model

Theory stating that reactions occur when particles collide with sufficient energy and proper orientation.

Activation energy (Ea)

The minimum energy required for a collision between reactant particles to result in a reaction.

Arrhenius equation

k = Ae^(-Ea/RT); relates the rate constant k to temperature T, activation energy Ea, and frequency factor A.

Frequency factor (A)

A term in the Arrhenius equation representing collision frequency and the fraction with correct orientation.

Linearized Arrhenius equation

ln(k) = -Ea/R(1/T) + ln(A); a plot of ln(k) vs 1/T gives a line with slope -Ea/R, used to determine Ea experimentally.

Try an AP-style question

Which two conditions must a collision meet to cause a reaction?

1. Enough energy and proper orientation
2. Equal masses and equal speeds
3. High pressure and low temperature
4. Enough mass and a catalyst